Re: [Qemu-devel] [RFC] Undeterministic behaviour with icount.

[Frederic Konrad]

On 10/07/2013 15:45, Paolo Bonzini wrote:
> Il 08/07/2013 19:32, Frederic Konrad ha scritto:
> > Hi everybody,
> >
> > We get some issues with reverse execution caused by indeterminism.
> >
> > Something catched our attention:
> > static void icount_warp_rt(void *opaque), cpus.c:276
> >
> > We have the feeling that icount is synchronized with rt_clock, is that
> > possible?
> When the CPU is idle, yes.
>
> Note that the same thing happened before the series you linked.  This is
> the relevant code:
>
> -        /* Wait for either IO to occur or the next
> -           timer event.  */
> -        add = qemu_next_deadline();
> -        /* We advance the timer before checking for IO.
> -           Limit the amount we advance so that early IO
> -           activity won't get the guest too far ahead.  */
> -        if (add > 10000000)
> -            add = 10000000;
> -        delta += add;
> -        qemu_icount += qemu_icount_round (add);
>
> It was adding to the icount while waiting for the next timer event to
> happen.
>
> > According to this Paolo's series
> > <http://lists.gnu.org/archive/html/qemu-devel/2011-04/msg01271.html>
> > this must be called when the cpus are sleeping, but
> > I saw this called frequently during the execution, is that the expected
> > behaviour?
> What workload are you running?  For anything that is not CPU bound I'd
> expect that to be the case.
>
> Paolo
>
> > If not what's the best way to fix that?
Hi Paolo,

Thanks for your answer.

For the moment the workload is a kernel boot. We removed everything from the
board.

So if it's the case, vm_clock will run non deterministic and we can't 
replay.

I just send a series about that.

Fred